gtkwindow: Remove resize grips

gtkwindow: Remove resize grips

Resize grips were introduced for GNOME 3.0, before we had any of the
"new GNOME app" features like invisible borders and CSD. With OS X 10.6
and 10.7, Apple has replaced the classic grips in their applications
with invisible borders as well.

New GNOME app designs don't use resize grips anymore and the new
default theme for GTK+, Adwaita, disables them entirely by forcing their
width and height to 0.

They're past their time. Remove the code to support them. This can
always be reverted if some app relies on them.